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| author | Thomas Gleixner <tglx@linutronix.de> | 2008-05-17 17:12:24 +0200 |
|---|---|---|
| committer | Thomas Gleixner <tglx@linutronix.de> | 2008-05-17 17:12:24 +0200 |
| commit | 538f0fd0f210c2ce5c585799f18d0e5c7cf6155e (patch) | |
| tree | e9fa2b10ce5d92ac6bcd8ac55af1cd97bda3ec5d /Documentation/memory-barriers.txt | |
| parent | 3bb6fbf9969a8bbe4892968659239273d092e78a (diff) | |
| parent | f26a3988917913b3d11b2bd741601a2c64ab9204 (diff) | |
Merge branch 'linus' into x86/garttip-x86-gart-2008-05-17-15-12-25
Diffstat (limited to 'Documentation/memory-barriers.txt')
| -rw-r--r-- | Documentation/memory-barriers.txt | 12 |
1 files changed, 11 insertions, 1 deletions
diff --git a/Documentation/memory-barriers.txt b/Documentation/memory-barriers.txt index e5a819a4f0c9..f5b7127f54ac 100644 --- a/Documentation/memory-barriers.txt +++ b/Documentation/memory-barriers.txt @@ -994,7 +994,17 @@ The Linux kernel has eight basic CPU memory barriers: DATA DEPENDENCY read_barrier_depends() smp_read_barrier_depends() -All CPU memory barriers unconditionally imply compiler barriers. +All memory barriers except the data dependency barriers imply a compiler +barrier. Data dependencies do not impose any additional compiler ordering. + +Aside: In the case of data dependencies, the compiler would be expected to +issue the loads in the correct order (eg. `a[b]` would have to load the value +of b before loading a[b]), however there is no guarantee in the C specification +that the compiler may not speculate the value of b (eg. is equal to 1) and load +a before b (eg. tmp = a[1]; if (b != 1) tmp = a[b]; ). There is also the +problem of a compiler reloading b after having loaded a[b], thus having a newer +copy of b than a[b]. A consensus has not yet been reached about these problems, +however the ACCESS_ONCE macro is a good place to start looking. SMP memory barriers are reduced to compiler barriers on uniprocessor compiled systems because it is assumed that a CPU will appear to be self-consistent, |